During processing of silver halide color photographic elements, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a "fixing" step. In some processes, the two steps can be combined in a so-called bleach-fix step.
Common bleaching agents include ferric chelate complexes of aminopolycarboxylate ligands, such as ethylenediaminetetraacetic acid (EDTA) and 1,3-propylenediaminetetraacetic acid (PDTA). These agents perform acceptably, but are not generally biodegradable, and environmental concerns are very prominent in many cultures. Other ligands have become acceptable for providing both bleaching and biodegradability, including .beta.-alaninediacetic acid (ADA), ethylenediaminedisuccinic acid (EDDS) and methyliminodiacetic acid (MIDA).
These biodegradable bleaching agents may have one or more deficiencies, most notably a lower bleaching rate than the EDTA complexes. Thus, ferric complexes of ADA are well known, but they are relatively slow bleaching agents compared to the ferric-EDTA complexes. Thus, they must be used in higher concentrations which is undesirable for cost and environmental reasons.
Japanese Kokai 51-07930 (published Jan. 22, 1976) describes the use of nitrilotriacetic acid or 2,6-pyridinedicarboxylic acid or both to reduce stains in neutralizing or fixing solutions. Bleaching solutions containing an aminocarboxylic acid metal complex salt or a polycarboxylic acid metal complex salt are also known. Japanese Kokai 53-048529 (published May 2, 1978) describes the use of such complexes to lower fog.
EP-A-0 329 088 (published Aug. 23, 1989) describes bidentate complexes in bleaching solutions which further contain buffers, one of which is 2-pyridinecarboxylic acid (PCA). Complexes of PCA with iron are not described.
A ferric complex of 2,6-pyridinedicarboxylic acid (PDCA) is described in U.S. Pat. No. 5,460,924 (Buchanan et al). Useful ternary bleaching agents are described in copending and commonly assigned U.S. Ser. No. 08/622,236 (filed Mar. 22, 1996, by Gordon et al). Such materials comprise one iron atom complexed with two different ligands.
Japanese Kokai 50-26542 (published Mar. 19, 1975) describes bleaching solutions containing a binary iron chelate with a ligand such as 2-carboxypyridine, 8-hydroxyquinoline or 2-carboxypyrazine. However, the molar ratios of these ligands to iron are quite low as demonstrated in the examples of that publication (less than 0.1:1).
PCA and PDCA are used as one ligand in ternary ferric bleaching agents as described in U.S. Pat. No. 5,521,056 (Buchanan et al). In such complexes, the PCA or PDCA must be present at a molar ratio to ferric ion of at least 0.6:1.
Besides slower bleaching rates, another problem encountered in some bleaching processes is the undesirable formation of a colorless or "leuco" form of cyan dyes within photographic elements, especially photographic color negative films, during processing. This may be observed particularly with 1-naphthol type cyan dyes, and thus there is a need to control it, or to actually convert the colorless form back to the colored dye (known as "leuco cyan dye recovery").
There remains a need in the art for highly water-soluble bleaching solutions which preferably contain biodegradable bleaching agents, that provide rapid and effective bleaching and are compatible with bromide or chloride rehalogenation. There is also a need for a way to reduce the formation of colorless forms of cyan dye, or to promote leuco cyan dye recovery, in bleached photographic elements.